An unusually long repeated DNA sequence was found 3' to the beta globin gene and at 3000-4800 other sites elsewhere in the human genome. DNA sequence analysis has shown that the copy of the 6.4 repeat family abuts exactly against an Alu I repeated sequence element. Further DNA sequencing is being done to look for other examples of such opposition. A library of recombinant M13 bacteriophage containing the human genome is being screened for DNA probes capable of detecting highly polymorphic sites in the human genome which could be useful in prenatal diagnosis of genetic diseases.